The basestations used by the providers of current day multiple channel wireless communication services, such as cellular mobile telephone (CMT) and personal communication systems (PCS), typically designate signal processing equipment for each single receiver channel. This is probably a result of the fact that each basestation is configured to provide communication capability for only a limited predetermined number of channels in the overall frequency spectrum that is available to the service provider.
A typical basestation may thus contain several racks of equipment which house multiple sets of receiver and transmitter signal processing components that service a prescribed subset of the available channels. For example, in an Advanced Mobile Phone Service (AMPS) cellular system, a typical basestation may service a pre-selected number of channels, such as 48, of the total number, such as 416, of the channels available to the service provider.
Certain types of wireless service providers would prefer, however, to employ equipment that would be more flexible, both in terms of where it can be located, as well as in the extent of the available bandwidth coverage provided by a particular transceiver site. This is particularly true in rural areas where cellular coverage may be concentrated along a highway, and for which the limited capacity of a conventional 48 channel transceiver may be inadequate. This may also be true in other instances, where relatively large, secure, and protective structures for multiple racks of equipment are not necessarily available or cost effective, such as for PCS applications.
One way to resolve this difficulty is to implement a basestation transceiver using a high speed analog-to-digital (A/D) converter and equipment which makes use of efficient digital filtering algorithms such as the Fast Fourier Transform (FFT), to separate the incoming signal energy into multiple ones of the desired channels. On the transmit side, this basestation implementation includes an inverse FFT processing combiner which outputs a combined signal representative of the contents of the communication channel signals processed thereby. In this manner, relatively compact, lightweight, inexpensive, and reliable digital integrated circuits may be used to cover the entire channel capacity offered by the service provider, rather than only the subset of the available channels.
Thus unlike prior art basestations, the wideband digital basestation is capable of receiving any channel. While this provides a certain number of advantages as described above, it also poses a number of unique problems to the service provider.
Perhaps most importantly, there exists a need to efficiently support a varying number of active channels and the required connections into the public switched telephone network.
These connections should be made in such a way as to simplify call control. Indeed, it would be desirable for as many of the call set up control functions required by such a basestation were handled to the maximum extent possible by the basestation itself.
By so simplifying the network interface, the Mobile Telephone Switching Office (MTSO) and/or Mobile Switching Center (MSC) through which the basestation is connected to the Public Switched Telephone Network (PSTN) may be freed, as much as possible, from the details of maintaining a proper connection from the PSTN to the remote subscriber unit.
Secondly, the basestation should make efficient use of the available resources to process each call. In particular, while the wideband channelizer separates the signals into channels, certain other signal processing resources such as demodulators and modulators are also required.
Using the wideband front end, any channel in the bandwidth available to the service provider is available at any time. However, it is desirable for such a basestation to only activate as many of the other, per-channel resources as is required to support the present call density.
By making the basestation's implementation of call processing resources as modular as possible, the basestation could initially be configured to support a limited number of channels. Then, as the demand for services grows, additional channels could be supported by the addition of the necessary resources.
In other instances, the basestation should be reconfigurable in the event of an change or expansion in one type of service. For example, given the emergence of several air interface standards such as code division multiple access (CDMA) as well as time division multiple access (TDMA) standards for cellular, it is desirable for a given wideband basestation to be able to support each such standard, thereby reducing the number of such basestations that need to be deployed. However, it would be desirable if the resources allocated to one particular air interface, when no longer needed, could then be made available to process signals formatted using the other air interface. That is, as the demands of one type of service or the other come and go, the basestation should be automatically reconfigured, without requiring an investment in new or different basestation resources.
Thus, several difficulties exist with a wideband digital basestation that can process at any time, any one of many channels in the RF bandwidth available to a service provider.